A recent trend in the field of tires for passenger cars is to place emphasis on fuel economy and wet grip performance for which a labelling system has been introduced. Tires for heavy duty trucks and buses are also increasingly required to meet the same performance demand.
Conventional techniques developed to achieve such required performance include incorporation of a modified styrene-butadiene rubber or a modified polybutadiene rubber into, use of a system containing silica and a silane coupling agent for, or incorporation of a silica having a large specific surface area into, a cap tread which comes into contact with the road surface. For example, the incorporation of silica into a cap tread can reduce rolling resistance and at the same time improve wet traction performance. Additionally, Patent Literatures 1 to 3 each disclose a method for improving fuel economy etc. by incorporating two types of silicas differing in nitrogen adsorption specific surface area or particle size into a tread. However, making improvements solely in a cap tread provides only a limited increase in fuel economy. In an attempt to enable tires to achieve even higher fuel economy, therefore, it has been common practice to introduce a technique for increasing fuel economy not only to a cap tread but also to a base tread located below the cap tread.
In order to enable tires to achieve even higher fuel economy, it is very effective to increase the proportion of the base tread to which a technique for increasing fuel economy has been introduced. However, too high a proportion of such a base tread in the whole tire tread may lead to exposure of the base tread on the tread surface in the final phase of tire wear, causing drastic decrease in abrasion resistance or wet grip performance. In view of these, a rubber composition is desired which is excellent in fuel economy and wet grip performance as well as abrasion resistance. However, it is very difficult to prepare a rubber composition having such physical properties, and a rubber composition for base treads in which fuel economy, wet grip performance, and abrasion resistance are highly improved while maintaining the balance between them has not been obtained yet.